The present invention relates to the production of water-soluble polyelectrolytes. The polymers that are produced are cationic vinyl polymers, which can then be used as flocculating agents in wastewater treatment, in ore and coal processing and in paper manufacture. In particular, the current process relates to an adiabatic gel polymerization process for the production of water-soluble polyelectrolytes using UV LED modules or a combination of a UV tube light sources and UV LED modules wherein the resulting polymer has increased throughput, lower residual monomer content as well as lower insolubles compared with current photoinitiated polymerization processes.
Water-soluble polyelectrolytes are used in large quantities worldwide in water treatment facilities to improve flocculation and dewatering of resulting sewage sludges. Generally, a cationic polymer is added to the system to bind with contaminants and dissolved particles in the water, such as, polymers of acrylic acid or its derivatives, methacrylic acid esters and copolymers of these esters from acrylamide.
In traditional water-solution polymerization, water-soluble monomers, such as acrylic monomers, can be polymerized in dilute aqueous solutions so as to obtain polymers in the form of gels which may then be dried and comminuted. When this is done, the polymerization can be carried out in a continuous process on a conveyor belt to which a layer of reactants is applied or discontinuously and batch-wise. The polyelectrolytes are generally synthesized by mixing a combination of monomer based on ethylenic unsaturated monomers and initiating a radical polymerization.
However, it was soon found that photoinitiated polymerization could be carried out with concentrated monomer solution and a high reaction rate at a lower reaction temperature, which is beneficial to the production of polymer with high molecular weight. Therefore, it is important that new photoinitiators and photoinitiator systems be developed to improve upon the heretofore developed products and processes.
The photoinitiators used in industry generally absorb light in the ultra-violet spectral range from around 250 nanometers to 450 nanometers, although a wide variety of photo-initiators absorbing from 400 nm to 700 nm have also been used. The photoinitiators convert this light energy into chemical energy in the form of reactive intermediates, such as free radicals, initiating polymerization.
Light absorption by the photoinitiator requires that an emission line from the light source overlaps with an absorption band of the photoinitiator. Therefore, it is realized that the photoinitiator is dependent upon the particular polymerization being accomplished.
Various compounds have been used in the past for the initiation of polymerization processes of monomers, whether cationic, anionic, or non-ionogenic. For example, diaryliodonium and triarylsulfonium salts are the most common compounds used as photoinitiators for cationic monomers. From the application point of view the most important physicochemical photoinitiators are their spectroscopic properties (i.e., the range and magnitude of light absorption) and the photo-cleavage efficiency (i.e., the efficiency of generation of a strong protic acids that initiate the cationic polymerization processes). These particular photoinitiators absorb UV light in the wavelength range between 220 nm and 280 nm, which creates an important technological problem, in that that there are no efficient and simultaneously powerful enough UV light sources that will emit in that range. Currently used and known light sources, which emit light in the absorption spectrum of the commercial cationic photoinitiators, are low pressure mercury lamps, deuterium bulbs are low-power light sources. Also seen are xenon lamps, which are broadband UV-Vis-NIR light sources and emit only a fraction of supplied energy below 300 nm. Therefore, in the photochemical industry, medium pressure mercury lamps (MPM lamps) are most commonly used as the source of ultraviolet light. However, these sources of light emit a big part of the energy at a broad range of wavelengths about 365 nm, which is away from the adsorption maximum of most commercial photoinitiators, resulting in lower product yields.
Photoinitiators, such as benzoin and benzoin derivatives have been used in the continuous production of polymers or copolymers of water-soluble monomers.
Current polymerization of cationic polyelectrolytes includes, in particular, polymerization of terpolymers of (meth)acrylamide, monomers based on cationic (meth)acrylic acid esters and monomers based on (meth)acrylamides and/or hydrolysis stable cationic monomers. The electrolytes are produced by known methods such as, emulsion, solution, gel and suspension polymerization.
However, there continues to be a need for new photoinitiated systems be developed for improvement upon the heretofore polymerized products and processes. Particularly, there is still a need wherein the resulting polymer has increased throughput, lower residual monomer content as well as lower insolubles compared with current photoinitiated polymerization processes.